Electrical utilities and principal power consuming industries have employed a variety of metering approaches to provide quantitative and qualitative evaluation of electrical power. The outputs provided by such metering systems vary to suit the particular needs of the user, selection of readouts generally being made from such parameter as volthours, volt.sup.2 hours, watthours, Qhours, varhours, VAhours, amperehours, and ampere.sup.2 hours. Certain of these quantities, for example, watt, var, Q, and VA may, for example, be designated as in or out depending upon the direction of current flow, the term "out" representing delivery to the user and the term "in" representing return of power to the generating entity.
Typically, a metering system monitors power supplied to isolation and scaling components to derive polyphase input representations of voltage and current. These basic inputs then are selectively treated to derive units of power and the like as above listed. The most extensively employed technique of power monitoring has been the measurement of watthours through the use of an electromechanical induction meter. However, such devices are limited and, thus, there have developed electronic analog techniques for carrying out multiplication and phase adjustment to achieve higher accuracies and a multitude of readouts.
Early analog approaches taken to provide power parameter outputs initially involve the use of thermally responsive coil elements and the like, the temperatures of which could be converted to outputs corresponding with power values. A lack of convenience and accuracy with such techniques led to interest in the utilization of Hall effect devices as multipliers wherein voltage-proportional generated magnetic fields and current were associated to provide a voltage output proportional to the product of current and voltage. Other devices have been developed which utilize an electronic arrangement serving to capitalize on the exponential transfer characteristic of solid-state devices to carry out multiplication. In general, these early analog multiplication techniques were somewhat unsatisfactory in exhibiting accuracies lower than desired as well as instability.
Another analog multiplier technique currently popular in the industry utilizes the system concept of time division multiplication. For example, the multiplier produces a pulse waveform whose amplitude is proportional to one variable, whose length relative to period is a function of another variable, and whose average value is proportional to the product of the two values. A variety of improvements in such time division multiplier circuits have been developed with respect to controlling phase and phase derived inaccuracies. Such improvements, for example, are described in U.S. Pat. No. 4,356,446, issued Oct. 16, 1982; and U.S. Pat. No. 4,408,283, issued Oct. 4, 1983; both assigned to the assignee of this invention.
Analog approaches to electrical parameter monitoring and multiplication techniques physically are beset with problems in achieving desired output accuracy. Accurate analog multiplier techniques physically are beset with problems in achieving desired output accuracy. Accurate analog multipliers are somewhat expensive and generally exhibit undesirable drift and component variations from device to device. Accordingly, a considerable amount of technique effort is required in their production and maintenance to provide for adjustment for these various inaccuracies. As a consequence of these deficiencies, other approaches have been contemplated by investigators. For example, should the line inputs by purely sinusoidal, then straightforward peak detection techniques would be available. However, line inputs experienced worldwide, while basically resembling sinusoids, exhibit substantial variations representing high and low frequency noise, high energy transients, and a multitude of variations. These variations generally are caused by any of a number of external phenomena, for example, rapidly changing loads developed from solid-state controllers such as silicon controlled rectifier driven devices. In effect, portions of the waveform may be essentially missing due to a high speed switching of loads. Additionally, metering systems may encounter a "creep" phenomenon at their inputs which essentially represents a low level signal amounting to noise.
Purely digital approaches to measuring electric power have been contemplated as ideal. With such an arrangement, for example, high rates of sampling may be employed and the instantaneous sample values then may be converted or digitized as binary values. This ideal approach generally has been considered to require very high speed systems which for some period of time were unavailable or of such cost and complexity as to preclude utilization for the instant purpose. As a compromise to the earlier unavailability of high speed sampling components, relatively slow sampling techniques, i.e. on the order of each 45.degree. of a cycle, have been proposed. To achieve some accuracy, the approaches then must employ an averaging of the sample values and thus, the advantage of high sample rate, instantaneous evaluation of waveform, and the like are not achieved, nor can the systems distinguish discrete vagaries in distorted sinusoids. A typical randomizing approach to digital metering is described, for example, in U.S. Pat. No. 4,077,061, issued Feb. 28, 1978.
A further design aspect which has impeded development of practical digital multiplication circuits resides in the somewhat limited range output of analog-to-digital conversion devices. Those available at practical cost, for example, provide 12 to 16 bit outputs which generally will be found to be inadequate to achieve the scale of accuracy desired by industry without some form of accommodation. In this regard, the conversion devices exhibit a form of truncated performance wherein the digital output thereof is predicated upon a given voltage threshold which may, in reality, fall between two successive digital values, a condition which the devices cannot accommodate.
One approach to achieving a high degree of accuracy using conventional analog-to-digital converters is described, for example, in U.S. Pat. No. 4,884,021, issued Nov. 28, 1989, and assigned in common herewith. The metering system described therein describes a technique for developing high conversion accuracy through a dual sampling technique wherein each sample of current and voltage is first submitted to a conversion to binary form for the purpose of developing a scaling evaluation and a scaling factor. The scaling evaluation is used to selectively adjust the gain of an amplification stage to which the electrical parameter for the sampled quantity is submitted prior to a second conversion. This second conversion then provides a data readout which is multiplied at high speed by the scaling factor to provide an expanded digital data value corresponding with the electrical parameters of voltage and current. The expanded values may, for example, have as high as 21 significant bits in conjunction with a sign bit. These expanded data then are selectively multiplied to develop digital representations for twelve power parameters. Through the use of dual conversion stages in combination with a zero cross-over monitoring of the input sinusoids, otherwise evasive electrical quantities such as volt amperes are readily developed through the metering approach by, in effect, carrying out an alignment of current and voltage sinusoids.
The accuracy of any digital metering approach not only is necessarily concerned with achieving adequate sampling and sufficiently accurate digital conversion, but also with the quality of each succeeding stage or function of the metering device. In this regard, errors may be introduced from a variety of aspects of meter design, for example in phase related components, multiplexing components, timing and final output or register stages. To achieve desired high accuracies, the performance of each such sample or data manipulation stage must be optimized for the generation of low error for outputs ranging from lower values to full scale.